Bug 1035763 -- Support 24 hour time format

This isn't actually testing what it says it is, but with a small tweak, it does.

The basic problem is that we're stubbing ```DateTimeFormat``` too soon, so it always returns '4:37 PM'. But we have nothing verifying that it wasn't already that before we started. So once we get to the verifications, all we're doing is verifying that it is '4:37 PM', but not that it changed.

You can fix this by moving the ```this.sinon.stub(MockMozL10n, 'DateTimeFormat', /* ... */)``` call into the ```appendAndCheckGroupDOM()``` callback.

I also discovered that you can remove this line from this file as it's not used:
```js
require('/dialer/test/unit/mock_l10n.js');
```